The present invention is related to electrostatic chucks for holding substrates in process environments.
Electrostatic chucks are used to hold substrates, such as silicon wafers, during processing of the substrate in a process chamber. A typical electrostatic chuck includes an insulated electrode that is electrically biased with respect to the substrate by an electrical voltage. During processing of the substrate, process gas is introduced into the process chamber, and in certain processes, an electrically charged plasma is formed from the process gas. In monopolar chucks the electrical voltage applied to the electrode and the charged plasma species above the substrate induce opposing electrostatic charge in the electrode and substrate that results in an attractive electrostatic force that electrostatically holds the substrate to the chuck. In bipolar chucks, two or more electrodes are maintained at different electrical potentials to electrostatically hold the substrate in both plasma and non-plasma processes.
Typically, the electrostatic chuck comprises a metal electrode covered by a thin insulator layer that maximizes electrostatic attractive forces. However, the thin insulator layer can be punctured by sharp edged substrate fragments, or eroded in the process environment to expose the electrode of the chuck. Such puncturing is particularly likely to occur when the insulator is composed of a soft polymeric material that has low puncture resistance. Exposure of the electrode at even a single pinhole in the insulator can cause arcing between the electrode and plasma and require replacement of the entire chuck. Failure of the electrostatic chuck during processing of a substrate can result in loss of the entire substrate at a significant cost.
One solution to the puncturing problem is to make the chuck resistant to failure from erosion of the insulator by the use of a plurality of fuses, each fuse electrically connecting at least one electrode in series to a power bus that supplies the voltage to the electrodes of the chuck, as disclosed in commonly assigned U.S. Pat. No. 5,751,357, by Kumar et al. The fuses burn-out and electrically disconnect the electrodes from the power bus when the insulator punctures and exposes the electrodes to the plasma process environment because of the plasma discharge current that flows through the fuses. However, because the fuses are embedded in the insulator layer, the burned-out fuses can result in a link of conductive residue that can form a weak conductive pathway between the electrode and the power bus. The residual conductive pathway allows the plasma to continue to discharge through the fuse, causing plasma instabilities in the chamber that result in non-uniform processing of the substrate and/or movement of the substrate by weakening the electrostatic force holding the substrate.
It is desirable to have an electrostatic chuck that is resistant to failure from puncturing by substrate fragments, and from failure by erosion in erosive process environments. It is also desirable for the chuck to be tolerant to punctures to allow the chuck to continue holding a substrate even with multiple punctures in the insulator layer. It is further desirable to have an electrostatic chuck that tolerates erosion or failure of portions of an insulator covering the electrode without allowing plasma currents to continue to discharge through the electrode.